Semiconductor device including ferroelectric capacitor

ABSTRACT

A semiconductor device includes a ferroelectric capacitor formed above the lower interlevel insulating film covering a MOS transistor formed on a semiconductor substrate, including lamination of a lower electrode, an oxide ferroelectric film, a first upper electrode made of conductive oxide having a stoichiometric composition AO x1  and an actual composition AO x2 , a second upper electrode made of conductive oxide having a stoichiometric composition BO y1  and an actual composition BO y2 , where y2/y1&gt;x2/x1, and a third upper electrode having a composition containing metal of the platinum group; and a multilayer wiring structure formed above the lower ferroelectric capacitor, and including interlevel insulating films and wirings. Abnormal growth and oxygen vacancies can be prevented which may occur when the upper electrode of the ferroelectric capacitor is made of a conductive oxide film having a low oxidation degree and a conductive oxide film having a high oxidation degree.

CROSS REFERENCE TO RELATED APPLICATION

This application is based on and claims priority of Japanese PatentApplication No. 2006-218924 filed on Aug. 10, 2006, the entire contentsof which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

A) Field of the Invention

The present invention relates to a semiconductor device and itsmanufacture method, and more particularly to a semiconductor devicehaving ferroelectric capacitors and its manufacture method.

B) Description of the Related Art

With the recent development of digital technologies, there arise highdemands for processing or storing a large amount of data at high speed.Therefore, semiconductor devices used in electronic apparatus arerequired to have high integration density and high performance. Forexample, in order to realize high integration density of dynamic randomaccess memories (DRAMs), techniques are widely researched and developedby which a ferroelectric material film or a high dielectric constantmaterial film is used as a capacitor dielectric film, in place of asilicon oxide film or a silicon nitride film.

Vigorous researches and developments have been conducted on aferroelectric memory (FeRAM) using a ferroelectric material film havingspontaneous polarization characteristics as a capacitor dielectric film,in order to realize a nonvolatile memory capable of data read/write atlower voltage and at high speed.

A ferroelectric memory stores information by utilizing hysteresischaracteristics of a ferroelectric capacitor having a ferroelectric filmsandwiched between a pair of electrodes. The ferroelectric filmgenerates polarization corresponding to a voltage applied between theelectrodes, and retains spontaneous polarization even if the appliedvoltage is removed. As the polarity of the applied voltage is reversed,the polarity of spontaneous polarization is also reversed. Informationcan be read by detecting spontaneous polarization. As compared to aflash memory, a ferroelectric memory operates at a lower voltage and canwrite data at high speed with reduced power dissipation.

A capacitor dielectric film of FeRAM is made of lead zirconate titanate(PZT), La-doped PZT (PLZT), PZT-containing material doped with Ca, Sr orSi slightly, Bi-containing layer structure compound such as SrBi₂Ta₂O₉(SBT) and SrBi₂(Ta, Nb)₂O₉ (SBTN) or the like, and manufactured by asol-gel method, sputtering, metal organic chemical vapor deposition(MOCVD) or the like.

Generally, an oxide ferroelectric film in an amorphous or micro crystalstate is formed on a lower electrode by the above-described film formingmethod, and thereafter heat treatment is performed to change the crystalstructure to a perovskite structure or a bismuth compound layerstructure. Therefore, the lower electrode of a ferroelectric capacitoris made of platinum group metal such as platinum (Pt) and iridium (Ir)hard to be oxidized in an oxygen atmosphere, or platinum groupconductive oxide such as iridium oxide (IrO_(x)). A ferroelectric filmis likely to form oxygen vacancies by heat treatment in an oxygenatmosphere so that characteristics of the ferroelectric film aredegraded, such as reduction in a switching charge and an increase in aleak current.

In manufacturing a ferroelectric capacitor, heat treatment in an oxygenatmosphere is required to be performed plural times in order to recovercrystal defects such as oxygen vacancies. Therefore, the upper electrodeof the ferroelectric capacitor is also made of platinum group metal suchas Pt and Ir hard to be oxidized in an oxygen atmosphere, or conductiveplatinum group oxide such as IrO_(x) and RuO_(x). Various proposals havebeen made using a laminated electrode of a plurality of electrodelayers.

JP-A-2003-174146 proposes that an upper electrode is made of a firstoxide electrode and a second oxide electrode, one being made of SrRuOwhich contains Pb, Bi, Cu at 0.1 at % or more and the other being madeof IrO_(x).

JP-A-2004-247324 proposes that a PbRuO conductive layer is formed on aPZT film and a RuO₂ conductive layer is formed on the PbRuO conductivelayer to form an upper electrode.

Also in FeRAM, there are high demands for miniaturization and lowvoltage operation in recent years. It is therefore required that aferroelectric film constituting a ferroelectric capacitor has asufficient switching charge quantity Q_(SW) even in a miniaturizedstructure. If a multilayer wiring structure is adopted, there is apossibility that the characteristics of a ferroelectric capacitor aredegraded by a reducing atmosphere process used by a multilayer wiringmanufacture method.

If an upper electrode is made of a Pt film, an Ir film or the like,there is a non-negligible danger that hydrogen in a reducing atmosphereused by a process of forming an interlevel insulating film invades thePt film or Ir film and is activated by a catalyst function of the metalfilm, and the activated hydrogen reduces the ferroelectric film. If theferroelectric film is reduced, the operation characteristics of theferroelectric capacitor are degraded considerably. This problem ofdegraded characteristics appears conspicuously particularly when theferroelectric capacitor is made micro fine.

JP-A-2002-324894 (U.S. Pat. No. 3,661,850) proposes that a first upperelectrode made of conductive oxide having a stoichiometric compositionAO_(x1) at a composition of AO_(x2) is formed on a ferroelectric film,and a second upper electrode made of conductive oxide having astoichiometric composition AO_(y1) at a composition of AO_(y2) is formedon the first upper electrode, where y2/y1>x2/x1. Namely, an oxidationdegree of the first upper electrode is suppressed to form a goodinterface with ferroelectric material, and the second upper electrode ismade of conductive oxide having an increased oxidation degree tosuppress generation of metal functioning as catalyst. AO_(y2) haspreferably a composition near the stoichiometric composition.

SUMMARY OF THE INVENTION

It has been found that a new problem occurs if the upper electrode of aferroelectric capacitor is made of a first conductive oxide film havinga suppressed oxidation degree and a second conductive oxide film havingan increased oxidation degree. An object of the second conductive oxidefilm is to shield hydrogen and H₂O (e.g., hydrogen and H₂O invading whenan interlevel insulating film is formed or when a W conductive layer isformed in a contact hole formed through the interlevel insulating film)invading from an upper region of the capacitor. In order to providesufficient function, a thickness of the second conductive oxide film isrequired to be 100 nm or thicker. However, as the second conductiveoxide film having a high oxidation degree becomes thick, abnormal growthis likely to occur. Namely, if a crystallized second conductive oxidefilm is formed thick, crystals are likely to grow abnormally on thesurface of the film. Particularly when the film is formed at hightemperature, the film having a thickness of 100 nm or thicker is likelyto grow abnormally.

As will be described later, in order to solve this problem, the secondconductive oxide film was formed just above the first conductive oxidefilm first at low temperature in an amorphous phase, thereafter acrystallized conductive oxide film was formed by changing a film formingpower. This second conductive oxide film has an amorphous phase in alower region, and an upper region is crystallized by a rise of thesubstrate temperature during film formation. Abnormal growth does notoccur while this second conductive oxide film is formed. However, thefollowing point has been found. The second conductive oxide film is veryunstable oxide, and is crystallized if the substrate is heated in laterprocesses or if heat treatment is performed. At the same time ofcrystallization, oxygen is removed from the second conductive oxidefilm, and oxygen vacancies are formed and a number of voids are formedin the film. Hydrogen and H₂O invading from the upper region of thecapacitor break the ferroelectric film via these voids, to degrade theelectric characteristics of the capacitor. Film stripping is likely tooccur during a multilayer wiring forming process.

An object of this invention is to provide a semiconductor device and itsmanufacture method capable of solving a newly found problem.

According to one aspect of the present invention, there is provided asemiconductor device comprising:

a semiconductor substrate;

a MOS transistor formed on the semiconductor substrate;

a lower interlevel insulating film covering the MOS transistor;

a ferroelectric capacitor formed above the lower interlevel insulatingfilm comprising:

a capacitor lower electrode;

an oxide ferroelectric film formed on the capacitor lower electrode;

a first capacitor upper electrode formed on the oxide ferroelectric filmand made of conductive oxide having a stoichiometric composition AO_(x1)and an actual composition AO_(x2);

a second capacitor upper electrode formed on the first capacitor upperelectrode and made of conductive oxide having a stoichiometriccomposition BO_(y1) and an actual composition BO_(y2), wherey2/y1>x2/x1; and

a third capacitor upper electrode formed on the second capacitor upperelectrode and having a composition containing metal of the platinumgroup; and

a multilayer wiring structure formed above the lower interlevelinsulating film, covering the ferroelectric capacitor and includinginterlevel insulating films and wirings.

According to another aspect of the present invention, there is provideda method of manufacturing a semiconductor device, comprising the stepsof:

(a) forming a MOS transistor on a semiconductor substrate;

(b) forming a lower interlevel insulating film on the semiconductorsubstrate, the lower interlevel insulating film covering the MOStransistor;

(c) forming a capacitor lower electrode above the lower interlevelinsulating film;

(d) forming a capacitor dielectric film of oxide ferroelectric materialon the capacitor lower electrode;

(e) forming a first capacitor upper electrode on the capacitordielectric film, the first capacitor upper electrode being made ofconductive oxide having a stoichiometric composition AO_(x1) and anactual composition AO_(x2);

(f) forming a second capacitor upper electrode on the first capacitorupper electrode, the second capacitor upper electrode being made ofconductive oxide having a stoichiometric composition BO_(y1) and anactual composition BO_(y2), where y2/y1>x2/x1; and

(g) forming a third capacitor upper electrode on the second capacitorupper electrode, the third capacitor upper electrode having acomposition containing metal of the platinum group, and

(h) forming a multilayer wiring structure above the lower interlevelinsulating film, covering the ferroelectric capacitor and includinginterlevel insulating films and wirings.

It is possible to suppress abnormal growth and generation of voids inthe upper electrode, during forming the upper electrode.

BRIEF DESCRIPTION OF THE DRAWINGS

FIGS. 1A to 1L are cross sectional views of a semiconductor substrateillustrating main processes of a method of manufacturing a semiconductordevice according to a first embodiment of the present invention.

FIGS. 2A to 2D are cross sectional views of a semiconductor deviceillustrating modifications of the first embodiment, and FIG. 2E is atable collectively showing measurement results.

FIGS. 3A to 3T are cross sectional views of a semiconductor substrateillustrating main processes of a method of manufacturing a semiconductordevice according to a second embodiment of the present invention.

FIGS. 4A and 4B are a SEM photograph showing a cross sectional structureof a ferroelectric capacitor manufactured by the second embodimentmethod, and a cross sectional view schematically showing the structure.

FIG. 5 is a cross sectional view of a semiconductor structureillustrating a third embodiment of the present invention.

FIG. 6 is a cross sectional view of a semiconductor structureillustrating a fourth embodiment of the present invention.

FIGS. 7A and 7B are a schematic cross sectional view showing aferroelectric capacitor structure used in preliminary experiments, and aSEM photograph showing the cross sectional structure of a prototypesample.

FIGS. 8A and 8B are a schematic cross sectional view showing the statethat a hard mask layer is formed on a ferroelectric capacitor structure,used in preliminary experiments, and a SEM photograph showing a crosssectional structure of a prototype sample.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Prior to description of embodiments of the present invention,preliminary experiments made by the inventor will be described. On thebasis of the proposal by JP-A-2002-324894, the upper electrode of aferroelectric capacitor was formed by laminating a first conductiveoxide film having a low oxidation degree, and a second conductive oxidefilm having a high oxidation degree and a composition near thestoichiometric composition on the first conductive oxide film. Thesecond conductive oxide film is a film for shielding hydrogen and H₂O(e.g., hydrogen and H₂O generated when an interlevel insulating film isformed or when a W conductive layer is formed in a contact hole formedthrough the interlevel insulating film) invading from an upper region ofthe capacitor. In order to provide sufficient function, a thickness ofthe second conductive oxide film is required to be 100 nm or thicker. Ithas been found, however, that as the second conductive oxide filmbecomes thick, abnormal growth is likely to occur. Particularly when thefilm is formed at high temperature, the film having a thickness of 100nm or thicker is likely to grow abnormally. In order to suppressabnormal growth, a trial was conducted to grow the second conductiveoxide film at low temperature and form a conductive oxide film in anamorphous phase.

As shown in FIG. 7A, on a TiAlN film 101 functioning as an oxygenbarrier layer, an Ir lower electrode 102 and a PZT ferroelectric film103 were formed, an IrO_(x) film 104 as a first upper electrode whoseoxidation degree was suppressed was formed on the PZT ferroelectricfilm, and then an IrO_(x) film 105 whose oxidation degree was increasedwas formed by reactive sputtering in an amorphous phase. Although anIrO_(x) 105 a in the amorphous phase was initially formed, acrystallized IrO_(x) film 105 b was then formed by a temperature riseduring film formation.

FIG. 7B is a SEM photograph showing the cross sectional structure at thestage when the lamination structure was formed. The first and secondIrO_(x) films are represented by IrO1 and IrO2, respectively. It seemsthat the state immediately after film formation poses no problem.

As shown in FIG. 8A, a TiN film 106 to be used as a hard mask was formedon the lamination structure. Heating during film formation crystallizedthe amorphous phase IrO_(x) film 105 a, formed holes or voids andchanged the IrO_(x) film 105 a to a film including voids or defects. Itcan be considered that oxygen was removed during crystallization andoxygen vacancies were formed. As a silicon oxide film to be used as ahard mask was formed by using tetraethoxysilane (TEOS) as sourcematerial, the voids or defects structure was emphasized more. Formingthe TiN film was performed at a temperature of, e.g., about 300° C., andforming the silicon oxide film was performed at a temperature of, e.g.,380° C. to 400° C.

FIG. 8B is a SEM photograph showing the cross sectional view of a statewherein holes or voids were formed. It can be observed that a number ofholes or voids 107 are formed and the film is changed to an IrO_(x) filmwith voids or defects. A film having these vacancies passes hydrogeneasily.

If a multilayer wiring forming process is executed in the state thatsuch vacancies exist, there is a high possibility that hydrogen and thelike invade and the electrical characteristics of a ferroelectriccapacitor are degraded. For example, a switching charge Qsw is halved.Stripping or peeling-off is likely to occur during the multilayer wiringforming process.

W plugs are widely used also in FeRAM. Use of W plugs is essentialparticularly for FeRAM having a high integration density. In forming a Wplug on a ferroelectric capacitor, a contact hole is formed, a TiN filmas a glue film is formed, and then a W film is formed on the TiN film.The W film is formed in a reducing atmosphere. Most of hydrogengenerated during film formation is blocked by the TiN film as the gluefilm. However, if excessive hydrogen is supplied, hydrogen may passthrough the blocking TiN film, and invade the upper electrode. IfIrO_(x) of the upper electrode is reduced, volume contraction of theupper electrode occurs and a gap is formed between the glue film andupper electrode. A contact resistance of the upper electrode may becomeunstable.

The present inventor has studied a method of suppressing abnormal growthof the upper electrode during film formation and suppressing generationof vacancies such as oxygen vacancies. Embodiments of the presentinvention will now be described.

The structure of FeRAM is roughly classified into a planar type in whicha ferroelectric capacitor is formed on an insulating film and contactsof the upper and lower electrodes are formed downward, and a stack typein which the lower electrode is connected to a conductive plug buried inan insulating film, a ferroelectric capacitor is formed on an insulatingfilm, and a contact of the upper electrode is formed downward.

With reference to FIGS. 1A to 1L, description will be made onmanufacture processes for a planar type FeRAM and a structure of themanufactured FeRAM, according to the first embodiment of the presentinvention.

As shown in FIG. 1A, for example, on the surface of a p-type Sisubstrate 1, an isolation region 2 is formed, for example, by localoxidation of silicon (LOCOS) to define active regions. The surfaces ofthe active regions are thermally oxidized to form a silicon oxide filmhaving a thickness of, e.g., about 10 nm to be used as a gate oxide filmGox. On the gate oxide film Gox, a polycide gate electrode G is formedwhich is a lamination of a first gate electrode G1 of polysilicon and asecond gate electrode G2 of silicide. By using a resist pattern, thelamination is patterned to form the gate electrode G, and thereafter,n-type impurity ions, e.g., As, are implanted shallowly to form lowconcentration extension regions Ex.

After the extension regions are formed, an insulating film of siliconoxide or the like is deposited by chemical vapor deposition (CVD), andanisotropic etching is performed by reactive ion etching (RIE) or thelike to remove the insulating film on a flat surface and leave sidewallspacers SW on the sidewalls of the gate electrode G. After the sidewallspacers SW are formed, n-type impurity ions, e.g., P are implanted toform high concentration source/drain regions S/D. In this way, a MOStransistor structure Tr is formed.

A silicon oxynitride film 11 is formed by CVD, covering the MOStransistor Tr. The silicon oxynitride film 11 has a function of abarrier film against invasion of hydrogen, oxygen and the like. On thesilicon oxynitride film 11, a silicon oxide film 12 as a lowerinterlevel insulating film is formed by CVD using TEOS. When necessary,the surface of the silicon oxide film 12 is planarized by chemicalmechanical polishing (CMP). A thickness of the silicon oxide film 12 isset to about 700 nm. Annealing is performed at 650° C. for about 30minutes in a N₂ atmosphere. Degassing (removal of moisture and the like)of the silicon oxide film 12 is therefore performed. On the siliconoxide film 12, an alumina (AlO) film 13 is formed by sputtering to athickness of about 20 nm. The alumina film is used as a tight adhesionfilm having a function of adhering the lower electrode of aferroelectric capacitor.

A Ti film or TiO_(x) film having a thickness of about 20 nm may be usedas the tight adhesion film.

On the tight adhesion film 13, a Pt film 26 as the lower electrode isformed to a thickness of about 150 nm, for example, by sputtering. Ifthe tight adhesion film 13 is made of Ti, a Ti film having a thicknessof about 20 nm is formed at a substrate temperature (film formingtemperature) of 150° C., and then a Pt film having a thickness of about180 nm is formed at a substrate temperature of 100° C. or 350° C.

As shown in FIG. 1B, on the lower electrode 26, a ferroelectric film 37is formed in an amorphous phase. For example, by using PLZT {(Pb, La)(Zr, Ti)O₃} as a target, a PLZT film having a thickness of about 100 nmto 200 nm is formed by RF sputtering. After the ferroelectric film 37 isformed, heat treatment is performed by rapid thermal annealing (RTA) ata temperature of 650° C. or lower, e.g., 560° C. in an atmosphere whichcontains Ar and O₂, and heat treatment is further performed by RTA at750° C. in an oxygen atmosphere. These heat treatments crystallizeperfectly the ferroelectric film 37. Furthermore, the Pt film as thelower electrode 26 is made dense so that mutual diffusion of Pt and Onear at the interface with the ferroelectric film 37 can be suppressed.

A first upper electrode 38 a is formed on the ferroelectric film 37. Forexample, an IrO_(x) film having a thickness of 20 nm to 50 nm is formedby reactive sputtering in the crystallization state when the film isformed. The film forming conditions are, for example, as in thefollowing.

-   -   Target: Ir,    -   Substrate temperature during film formation: 300° C.,    -   Film forming work gas: Ar+O₂,    -   Flow rate: [Ar]=140 sccm, [O₂]=60 sccm,    -   Flow rate ratio: [O₂]/[Ar]=0.43,    -   Sputtering power: about 1 kW to 2 kW.

IrO_(x) of the first conductive oxide film 38 a formed by reactivesputtering has a composition whose oxygen composition x is smaller thanthat of the stoichiometric composition (x=2).

As shown in FIG. 1C, an IrO_(y) film having a thickness of 30 nm to 100nm as a second conductive oxide film 38 b is formed on the firstconductive oxide film 38 a by reactive sputtering. The film formingconditions are, for example, as in the following.

-   -   Target: Ir,    -   Substrate temperature during film formation: 300° C.,    -   Film forming work gas: Ar+O₂,    -   Flow rate: [Ar]=100 sccm, [O₂]=100 sccm,    -   Flow rate ratio: [O₂]/[Ar]=1.00,    -   Sputtering power: about 1 kW to 2 kW.

IrO_(y) of the second conductive oxide film 38 b did not growabnormally, but a uniformly crystallized film was obtained. IrO_(y) ofthe second conductive oxide film 38 b has an oxygen composition y higherthan the oxygen composition x of the first conductive oxide filmIrO_(x), y>x, and nearer to the stoichiometric composition.

As shown in FIG. 1D, on the second conductive oxide film 38 b ofIrO_(y), a third conductive oxide film 38 c of IrO_(z) having athickness of 50 nm to 150 nm is formed. The film forming conditions are,for example, as in the following.

-   -   Target: Ir,    -   Substrate temperature during film formation: 300° C.,    -   Film forming work gas: Ar+O₂,    -   Flow rate: [Ar]=160 sccm, [O₂]=40 sccm,    -   Flow rate ratio: [O₂]/[Ar]=0.25,    -   Sputtering power: about 1 kW to 2 kW.

Since the oxygen flow rate is lowered, IrO_(z) of the third conductiveoxide film 38 c has a low oxygen composition z, z<y, z<x. The conductiveoxide film having a high metallic composition has no abnormal growth. Avery uniformly crystallized film was obtained.

A film forming temperature of the second conductive oxide film 38 b andthird conductive oxide film 38 c is not limited to 300° C., but may beselected from a range of, e.g., 100° C. to 400° C. The IrO_(z) ispreferably formed in the crystallization state when the film is formed.To this end, the film forming temperature is preferably 100° C. orhigher. If the film forming temperature is raised to 400° C. or higher,abnormal growth is likely to occur and it is difficult to form oxide. Itis therefore preferable to set the temperature to 400° C. or lower. Aflow rate and flow rate ratio of film forming work gasses may beselected properly in accordance with a film forming temperature.

In place of the third conductive oxide film, a film of a metal of theplatinum group may be used such as an Ir film and a Ru film. If an Irfilm is to be formed, this film can be formed by sputtering, forexample, at 400° C. at an Ar flow rate of 199 sccm. A combination of thefirst upper electrode 38 a, second upper electrode 38 b and third upperelectrode 38 c constitutes the upper electrode 38 of the ferroelectriccapacitor.

In this embodiment, the PLZT ferroelectric film 37 is formed bysputtering and crystallized by RTA at a temperature of, e.g., 560° C.followed by RTA at a temperature of, e.g., 750° C., and the upperelectrode is formed on the PLZT ferroelectric film. Severalmodifications of this manufacture method are possible. Thesemodifications will now be described.

As shown in FIG. 2A, in the first and second modifications, similar tothe first embodiment, after a ferroelectric film 37 is formed, RTA isperformed at 650° C. or lower, e.g., 560° in an atmosphere whichcontains Ar and O₂, and the following RTA at 750° C. in O₂ is omitted.

As shown in FIG. 2B, on the ferroelectric film 37, an IrO_(x) film 38 ahaving a thickness of 20 nm to 75 nm is formed by reactive sputtering ata room temperature (first modification) or at high temperature (secondmodification). In the first modification (room temperature filmformation), the film forming conditions are, for example, an Ar flowrate of 100 sccm, an O₂ flow rate of 56 sccm and a film forming power of2 kW. In the second modification (high temperature film formation), thefilm forming conditions are, for example, a substrate temperature of300° C., an Ar flow rate of 140 sccm, an O₂ flow rate of 60 sccm and afilm forming power of 1 kW. Thereafter, heat treatment is performed at atemperature of 650° C. to 750° C., e.g., 725°. This heat treatmentcrystallizes perfectly the ferroelectric film 37. At the same time, theinterface between the ferroelectric film 37 and first conductive oxidefilm 38 a becomes flat. These results are advantageous for a low voltageoperation and switching of the ferroelectric capacitor.

FIG. 2C shows the third modification. After the ferroelectric film 37 isformed in the similar manner to that described above, heat treatment isperformed at a temperature of, e.g., 560° C. to crystallize the film.Thereafter, a thin amorphous ferroelectric film 37 x is formed on theferroelectric film 37. On the thin ferroelectric film 37 x, a firstconductive oxide film is formed by the similar manner to that describedabove, and heat treatment is performed. As the amorphous ferroelectricfilm is laminated after the ferroelectric film is formed once, it ispossible to obtain the advantage of reducing leak current of thecapacitor. The ferroelectric film 37 may be formed in a crystallizationstate. In this case, without heat treatment, the amorphous ferroelectricfilm 37 x is formed thereon.

FIG. 2D shows the fourth modification. After the first conductive oxidefilm 38 a and second conductive oxide film 38 b are formed, heattreatment is performed at 650° C. to 750° C., e.g., 700° C. This heattreatment improves tight adhesion between the upper electrode andferroelectric film and crystallinity of the upper electrode. Descriptionwill be reverted to the first embodiment.

As shown in FIG. 1E, back side cleaning is performed to removeattachments during the film forming processes. Thereafter, the upperelectrode, ferroelectric film and lower electrode are etched in tiers orstepwise. First, a resist pattern is formed on the upper electrode 38and the upper electrode is patterned by dry etching. After etching, theresist pattern is removed and an annealing process is executed at 650°C. for 60 minutes in an oxygen atmosphere. Physical damages caused inthe ferroelectric film 37 by the dry etching can therefore be recovered.Next, a resist mask having a desired pattern of the ferroelectric filmis formed and the ferroelectric film 37 is patterned. Next, resist maskis removed, and oxygen annealing is performed.

As shown in FIG. 1F, an alumina film 41 is formed by sputtering,covering the patterned ferroelectric film 37 and upper electrode 38. Thealumina film has a function of preventing invasion of hydrogen. Oxygenannealing is performed to relax damages caused by sputtering.

As shown in FIG. 1G, by using a resist pattern, the alumina film 41 andlower electrode 26 are patterned. After etching, the resist pattern isremoved and oxygen annealing is performed.

As shown in FIG. 1H, an alumina film 42 is further formed by sputtering,covering the patterned alumina film 41. Next, oxygen annealing isperformed. The oxygen annealing before this alumina film is formed iseffective for preventing stripping or peeling-off of the alumina film.The oxygen annealing after the last alumina film was formed is effectivefor reducing capacitor leak current. The MOS transistor andferroelectric capacitor are formed in this way, which are mainconstituent elements of FeRAM. Wirings between the constituent elementsare still not formed.

As shown in FIG. 1I, an interlevel insulating film 43 of silicon oxidehaving a thickness of, e.g., about 1.5 μm is formed on the wholesubstrate surface by high density plasma (HDP) CVD. Next, the interlevelinsulating film 43 is planarized by CMP. A plasma process using N₂O gasis executed to nitridize the surface of the interlevel insulating film43. The nitridized surface becomes a barrier against permeation ofmoisture. Instead of the N₂O plasma process, a plasma process may beperformed which contains at least one of N and O.

As shown in FIG. 1J, a contact hole reaching one of the source/drainregions S/D of the MOS transistor Tr is formed through the interlevelinsulating film 43, alumina films 42 and 13, silicon oxide film 12 andsilicon oxynitride film 11. A Ti film and then a TiN film are formed bysputtering. These films constitute a barrier metal film. Then, a W filmis formed by CVD to be buried in the contact hole. Unnecessary W film,TiN film and Ti film deposited on the flat surface are removed by CMP toform a W plug 27. A SiON film 44 is formed covering the W plug 27. TheSiON film 44 functions as an oxidation preventive film for the W plug27.

As shown in FIG. 1K, contact holes are formed through the SiON film 44and interlevel insulating film 43 and further alumina films to exposethe upper electrode 38 and lower electrode 26 of the ferroelectriccapacitor. After the contact holes are formed, oxygen annealing isperformed to recover damages and degas moisture and the like in theinterlevel insulating film.

As shown in FIG. 1L, the SiON film 44 is removed by etch-back to exposethe surface of the W plug 27. Next, an Al wiring layer is formed andpatterned to form alumina wirings 28. In the structure shown in thefigure, the lower electrode 26 of the ferroelectric capacitor isconnected to one of the source/drain regions S/D of the MOS transistorvia the Al wiring 28 and W plug 27.

Forming an interlevel insulating film and wirings is repeated. Contactplugs are formed in interlevel insulating films where necessary. Afterwirings are formed, a silicon oxide film formed by TEOS and a cover filmof SiN are formed. In this way, a FeRAM device is manufactured.

By using the methods of the first embodiment and its modifications,various types of iridium oxide films were formed, and stoichiometry wasmeasured with a high resolution Rutherford backward scattering (RBS)analyzer HRBSV500.

FIG. 2E is a table collectively showing the measurement results. IrO_(x)indicates the first iridium oxide film. IrO_(y) indicates the secondiridium oxide film. IrO_(z) indicates the third iridium oxide film.

In the case of the film forming temperature of 300° C. (the firstembodiment), the first iridium oxide film IrO_(x) formed at the[Ar]:[O₂] flow rate ratio of 140:60 has an oxygen composition x=1.92 andis in the oxygen shortage state from the stoichiometric composition(x=2). The second iridium oxide film IrO_(y) formed at the [Ar]:[O₂]flow rate ratio of 100:100 has an oxygen composition y=2.02 and is inthe slightly excessive oxygen state almost equal to the stoichiometriccomposition (y=2). The second iridium oxide film IrO_(y) formed bylowering slightly the [Ar]:[O₂] flow rate ratio of 120:80 has an oxygencomposition y=2.00 just equal to the stoichiometric composition. (y=2).The third iridium oxide film IrO_(z) formed at the [Ar]:[O₂] flow rateratio of 160:40 has an oxygen composition z=1.84 which is the highestdegree of oxygen shortage from the stoichiometric composition (z=2). Ifthe film is formed at the same temperature and the same total flow rate,the oxygen composition is considered to be determined almost by the flowrate ratio [O₂]/[Ar].

In the case of the film forming temperature of 20° C. (the firstmodification), the first iridium oxide films IrO_(x) formed at the[Ar]:[O₂] flow rate ratios of 100:52 and 100:59 have oxygen compositionsx=1.20 and 1.50, respectively. Although the flow rate ratio [O₂]/[Ar] ishigher than that of film formation at 300° C., the oxygen composition xis lower. Since the conditions are changed by changing the total flowrate, simple comparison should not be made and it is desired to confirmthe tendency experimentally. The second iridium oxide film IrO_(y)formed at the [Ar]:[O₂] flow rate ratio of 100:100 has an oxygencomposition y=2.10. It is considered that the excessive oxygen state isincreased more than that of film formation at 300° C.

The oxygen compositions are x=1.92, y=2.02 and z=1.84 at the filmforming temperature of 300° C. and at the flow rate ratio of the firstembodiment. The oxygen compositions of the first and third iridium oxidefilms are lower than the stoichiometric composition, and the secondiridium oxide film has an oxygen composition almost equal to thestoichiometric composition. An oxidation degree of the second IrO_(y)film is higher than the first IrO_(x) film of the upper electrode, andhas an oxygen composition almost equal to the stoichiometriccomposition. An oxidation ratio of the third IrO_(z) film is lower thanthe second IrO_(y) film and lower than the first IrO_(x) film.

Indicating the real composition with a suffix 1, and the stoichiometriccomposition with a suffix 2, x₁=1.92 and x₂=2.00 makes the relativecomposition or ratio r_(x)=x₁/x₂=1.92/2.00=0.96. It may be consideredthat the ratios are preferably r_(x)≦0.96, r_(y)>0.96, and r_(z)≦0.96.

In the above description, the upper electrode is made of IrO. Since thesame material is used, the oxidation degree can be compared by usingonly the oxygen compositions x, y and z. If different oxide materialsare used, similar analysis may be made by using ratios of oxygencompositions x2, y2 and z2 to stoichiometric compositions of thematerial x1, y1 and z1, i.e., ratios x2/x1, y2/y1 and z2/z1.

The first conductive oxide film of the upper electrode is preferablymade of a film which is suitable for forming an interface with theferroelectric layer and having a suppressed oxidation degree. The secondconductive oxide film on the first conductive oxide film is formed tohave a higher oxidation degree (near the stoichiometric composition) anda limited thickness. By forming the second conductive oxide film by afilm having an oxygen composition near the stoichiometric composition),it becomes possible to suppress metallic state composition, activationof hydrogen and reduction of the ferroelectric film. By limiting thethickness, it becomes possible to suppress abnormal growth. Formation ofan interface layer at the interface between the first conductive oxidefilm and ferroelectric film can be suppressed so that growth of giantcrystals can be suppressed. By forming the third conductive oxide filmhaving a lower oxidation degree, characteristics suitable for the upperelectrode of the ferroelectric capacitor can be obtained. Goodferroelectric capacitor characteristics can therefore be obtained.

It is known that Ir and Pt in a metallic state function as catalyst forhydrogen. When Ir and Pt in the metallic state contact hydrogen, thehydrogen is activated. Therefore, if a single layer Ir or Pt film isused as the upper electrode, process deterioration of the capacitor islikely to occur. Similar process deterioration is likely to occur evenif an Ir or Pt film in the metallic state is formed on the firstconductive oxide film. For example, after a three-layer wiring structureis formed, the switching charge of the capacitor lowers to 50% of thatbefore wiring formation. When the first conductive oxide film is made ofIrO_(x) (x=1.3 to 1.8), the interface with the ferroelectric film isgood. Since the composition x is smaller than the stoichiometriccomposition of 2.0, it can be considered that oxygen vacancies areformed and oxide contents and metallic state contents exist mixedly. Ashydrogen invades such a film, hydrogen may be activated by metallicstate contents and the characteristics of ferroelectric oxide may bedegraded. As the second conductive oxide film IrO_(y) (y=2) having theoxygen composition near the stoichiometric composition is formed on thefirst conductive oxide film, hydrogen is hard to be activated becausethe second conductive oxide film has less oxygen vacancies and containsalmost no metallic state contents. It can therefore be considered thateven after a multilayer wiring structure is formed above theferroelectric capacitor, the capacitor characteristics can be retainedwithout deterioration.

According to the embodiment described above, the interface between theupper electrode and ferroelectric film can be improved and the processdeterioration can be suppressed. As a result, it is possible to improvethe switching charge, lower the coercive voltage and improve fatigueresistance and imprint resistance. Although the planar type FeRAM hasbeen described above, a stack type FeRAM may also be formed.

With reference to FIGS. 3A to 3T, description will be made on a stacktype FeRAM manufacture method according to the second embodiment of thepresent invention.

As shown in FIG. 3A, an isolation trench is formed in a surface layer ofa substrate 1 of n- or p-type silicon. After the trench surface isoxidized, a silicon oxide film is buried, for example, by high densityplasma (HDP) CVD, and unnecessary regions are removed by CMP to form anisolation region 2 by shallow trench isolation (STI) and define activeregions. The isolation region may be formed by LOCOS instead of STI. Ap-type well 3 is formed by implanting p-type impurity ions into asurface layer of the active region.

A common source region of two MOS transistors is connected to a bitline, and a capacitor is connected to each drain to form two memorycells. This interconnect arrangement is widely used in DRAM. Theembodiment also adopts this interconnect arrangement. Two MOStransistors 5 are formed in the active region. Brief description willnow be made on a method of forming the MOS transistor 5.

A surface layer of the active region is thermally oxidized to form aSiO₂ film as a gate insulating film. A silicon film of amorphous siliconor polysilicon is formed on the substrate and patterned to form a gateelectrode 5G. Two gate electrodes traverse one active region generallyin parallel.

By using the gate electrode 5G as a mask, n-type impurity ions areimplanted to form extension regions of a source region 5S and a drainregion 5D. Sidewall spacers of silicon oxide or the like are formed onthe sidewalls of the gate electrode 5G. By using the gate electrode 5Gand sidewall spacers as a mask, n-type impurity ions are implanted toform deep and high concentration regions of the source region 5S anddrain region 5D. With these processes, the MOS transistor 5 is formed.

Next, a film of refractory metal such as cobalt (Co) is formed on thesubstrate by sputtering. Heat treatment is performed to react therefractory metal film with silicon to form a refractory metal silicidelayer on the upper surfaces of the gate electrode 5G, source region 5Sand drain region 5D. Thereafter, unreacted refractory metal films areremoved, and heat treatment is performed when necessary to form silicidefilms 6.

A cover insulating film 11 of SiON having a thickness of 200 nm isformed on the substrate by plasma CVD, covering the MOS transistors 5.An interlevel insulating film 12 of SiO₂ having a thickness of 1000 nmis formed on the cover insulating film 11. For example, the interlevelinsulating film 12 is formed by plasma CVD using oxygen (O₂) andtetraethoxysilane (TEOS). Thereafter, the surface of the interlevelinsulating film 12 is planarized by chemical mechanical polishing (CMP).CMP is controlled so that the interlevel insulating film on the flatsurface of the substrate has a thickness of about 700 nm afterplanarization.

Via holes are formed through the interlevel insulating film 12 and coverinsulating film 11, reaching the silicide film 6 on the drain region 5Dand the silicide film 6 on the source region 5S. The via hole has adiameter of, e.g., 0.25 μm.

The inner surfaces of the via holes and an upper surface of theinterlevel insulating film 12 are covered with two layers, a Ti filmhaving a thickness of 30 nm and a TiN film having a thickness of 20 nm.A W film is formed on the two layers until the insides of the via holesare buried completely. A thickness of the W film is set to, e.g., 300nm. Unnecessary W film, TiN film and Ti film are removed by CMP. A tightadhesion layer constituted of the Ti films, TiN films and conductiveplugs 15 and 16 made of the W film are therefore left in the via holes.The conductive plugs 15 and 16 are connected to the drain region 5D andsource region 5S, respectively.

As shown in FIG. 3B, an oxidation preventive film 21 of SiON having athickness of 130 nm is formed on the interlevel insulating film 12 byplasma CVD. Instead of SiON, the oxidation preventive film 21 may bemade of SiN or AlO. An interlevel insulating film 22 of SiO₂ having athickness of 300 nm is formed on the oxidation preventive film by plasmaCVD using O₂ and TEOS.

As shown in FIG. 3C, via holes are formed through the interlevelinsulating film 22 and oxidation preventive film 21 to expose theunderlying conductive plugs 15. The inner surface of the via hole iscovered with a tight adhesion layer and W is buried in the via hole toform conductive plugs 25. This conductive plug 25 is formed by similarmethod as that used for the underlying conductive plug 15. Instead ofthe W plug, a polysilicon plug may also be used.

CMP for removing unnecessary W film and tight adhesion film is performedunder the conditions that a polishing speed of the W film and tightadhesion film is faster than that of the interlevel insulating film 22.For example, SSW2000 manufactured by Cabot Microelectronics Corporationis used as slurry. Slight over-polishing is performed so as not to leavethe tight adhesion film and W film on the interlevel insulating film 22.Therefore, the upper surface of the conductive plug 25 becomes lowerthan the upper surface of the surrounding interlevel insulating film 22to form a depression 25 a. A depth of the depression 25 a is, e.g., 20nm to 50 nm, typically about 50 nm.

After CMP, the upper surface of the interlevel insulating film 22 andthe upper surfaces of the conductive plugs 25 are exposed to plasma ofammonium (NH₃). This plasma process is executed, for example, under thefollowing conditions by using a parallel plate plasma processing system.

-   -   Distance between a substrate surface and an opposing electrode:        about 9 mm (350 mils);    -   Pressure: 266 Pa (2 Torr);    -   Substrate temperature: 400° C.;    -   NH₃ gas flow rate: 350 sccm;    -   RF power of 13.56 MHz supplied to a substrate side electrode:        100 W;    -   RF power of 350 kHz supplied to the opposing electrode: 55 W;    -   Process time: 60 seconds.

With this NH₃ plasma process, NH radicals are coupled to oxygen atoms onthe surface of the silicon oxide film.

As shown in FIG. 3D, a Ti film having a thickness of 100 nm is formed onthe surface subjected to the plasma process, by DC sputtering. Forexample, the sputtering conditions are as in the following.

-   -   Target: Ti;    -   Distance between a substrate and a target: 60 mm;    -   Ar gas pressure: 0.15 Pa;    -   Substrate temperature: 20° C.;    -   Sputter power: 2.6 kW;    -   Film forming time: 35 seconds.

Since NH radicals are coupled to oxygen atoms on the surface of thesilicon oxide film, Ti atoms attached to the surface can freely migrateon the surface without being captured by oxygen atoms. A Ti film havinga hexagonal closest packing structure and a self-organized (0 0 2)orientation is formed on the surface of the interlevel insulating film.In this sense, the silicon oxide film modified with NH radicals can besaid as a crystallinity improving film.

Next, rapid thermal annealing (RTA) is performed in a nitrogenatmosphere. For example, the RTA conditions are as in the following.

-   -   Annealing temperature: 650° C.;    -   Process time: 60 seconds.

With this annealing, the Ti film is nitridized and an underlyingconductive film 30 can be formed having a face centered cubic latticestructure and made of (1 1 1) oriented TiO. A thickness of theunderlying conductive film 30 may be set in a range of 100 nm to 300 nm.At this stage, a depression is formed on the surface of the underlyingconductive film 30 above the conductive plug 25, by reflecting thedepression 25 a on the underlying surface. The surface of the underlyingconductive film 30 is planarized by CMP. For example, SSW2000manufactured by Cabot Microelectronics Corporation is used as slurry. Athickness of the underlying conductive film 30 after CMP is 50 nm to 100nm, typically about 50 nm.

Crystals near the surface of the underlying conductive layer subjectedto CMP have strain caused by polishing. If the lower electrode of aferroelectric capacitor is formed on this underlying conductive layer,strain of the underlying conductive layer is transferred to the lowerelectrode, crystallinity of the lower electrode and further the upperferroelectric film is adversely affected. In order to avoid this, afterCMP, the surface of the planarized underlying conductive film 30 isexposed to NH₃ plasma. Crystal strain formed in a surface layer of theunderlying conductive film 30 can therefore be recovered.

As shown in FIG. 3E, a Ti film having a thickness of 20 nm is formed bysputtering on the underlying conductive film 30 whose crystal strain wasrecovered by NH₃ plasma. This Ti film is a crystalline conductive filmfunctioning as a tight adhesion film. RTA is performed in a nitrogenatmosphere. For example, the RTA conditions are as in the following.

-   -   Annealing temperature: 650° C.;    -   Process time: 60 seconds.

With this annealing, the Ti film is nitridized and an crystallinityimproved film 31 can be formed having a face centered cubic latticestructure and made of (1 1 1) oriented TiN.

As shown in FIG. 3F, an oxygen barrier film 33 of TiAlN having athickness of 100 nm is formed on the crystallinity improved film 31 byreactive sputtering using a TiAl alloy target. For example, thesputtering conditions are as in the following.

-   -   Ar gas flow rate: 40 sccm;    -   N₂ gas slow rate: 10 sccm;    -   Pressure: 253.3 Pa;    -   Substrate temperature: 400° C.;    -   Sputter power: 1.0 kW.

A lower electrode 36 of Ir having a thickness of 100 nm is formed on theoxygen barrier film 33 by sputtering. For example, the sputteringconditions are as in the following.

-   -   Ar atmosphere pressure: 0.11 Pa;    -   Substrate temperature: 500° C.;    -   Sputter power: 0.5 kW.

After the lower electrode 36 is formed, RTA heat treatment is performedin an Ar atmosphere at a temperature higher than a film formingtemperature of the lower electrode 36. Specifically, RTA is performedunder the following conditions.

-   -   Temperature: 650° C.;    -   Process time: 60 seconds.

With this heat treatment, crystallinity of the lower electrode can beimproved. In-plane distribution of crystallinity can also be improved.With this heat treatment, a constituent element Al of the oxygen barrierfilm 33 reacts with a constituent element Ir of the upper electrode 36,and an intermediate layer 34 of IrAl alloy is formed at the interfacetherebetween. The intermediate layer 34 improves tight adhesion betweenthe oxygen barrier film 33 and upper electrode 36. An atmosphere of theheat treatment may be other inert gas such as nitrogen and He, insteadof Ar. The lower electrode may be made of platinum group metal such asIr and Pt, conductive oxide such as PtO, IrO_(x) and SrRuO₃ or alamination of these films. If the lower electrode 36 is made of Pt orPtO, an intermediate layer 34 is formed which contains PtAl alloy. Ifthe lower electrode 36 is made of SrRuO₃, an intermediate layer 34 isformed which contains RuAl alloy.

As shown in FIG. 3G, a ferroelectric film 37 of PZT is formed on thelower electrode 36 by metal organic chemical vapor deposition (MOCVD).Description will now be made on a method of forming the ferroelectricfilm 37.

As Pb source material, liquid source material having a concentration of0.3 mol/l is used which is obtained by dissolving Pb(C₁₁H₁₉O₂)₂[Pb(DPM)2] in tetrahydrofuran (THF). As Zr source material, liquidsource material having a concentration of 0.3 mol/l is used which isobtained by dissolving Zr(C₉H₁₅O₂)₄ [Zr(dmh d)4] in THF. As Ti sourcematerial, liquid source material having a concentration of 0.3 mol/l isused which is obtained by dissolving Ti(C₃H₇O)₂(C₁₁H₁₉O₂)₂[Ti(O-iOr)2(DPM)2] in THF. These liquid source materials together withTHF solvent of 0.474 ml/min is supplied to a vaporizer of a MOCVDsystem. Flow rates of Pb source material, Zr source material and Tisource material are 0.326 ml/min, 0.200 ml/min and 0.200 ml/min,respectively.

The substrate on which the ferroelectric film 37 is formed is loaded ina chamber of the MOCVD system. A pressure in the chamber is set to 665Pa (5 Torr) and a substrate temperature is set to 620° C. Vaporizedsource material gasses are supplied to the inside of the chamber, andfilm formation is performed for 620 seconds. A PZT film having athickness of 100 nm is therefore formed.

Next, a second PZT film in an amorphous phase is formed by sputtering toa thickness of 1 nm to 30 nm, typically 20 nm. By locating the PZT filmin the amorphous phase, leak current can be reduced. An amorphousferroelectric film may be formed by MOCVD.

As shown in FIG. 3H, an upper electrode 38 is formed on theferroelectric film 37 under the same conditions as those of the firstembodiment, the upper electrode having a three-layer structureconstituted of a first conductive oxide film 38 a, a second conductiveoxide film 38 b and a third conductive oxide film 38 c. For example, byusing the process similar to the first embodiment, an IrO_(x) film isformed by reactive sputtering to a thickness of 20 nm to 70 nm, in thecrystallization state when the film is formed, the IrO_(x) film havingan oxygen composition x lower than the stoichiometric composition (x=2).

After the first conductive oxide film 38 a is formed, RTA is performedunder the following conditions.

-   -   Process temperature: 725° C.;    -   Atmosphere: O₂ flow rate 20 sccm+Ar flow rate 2000 sccm;    -   Process time: 60 seconds.

With this heat treatment, the ferroelectric film 37 is crystallizedperfectly, and at the same time, damages can be recovered which werecaused by exposure of the PZT film 37 to plasma when the firstconductive oxide film 38 a was formed, and oxygen vacancies in the PZTfilm can be compensated.

Formed on the first conductive oxide film 38 a by processes similar tothose of the first embodiment are a second conductive oxide film(IrO_(y) film) 38 b having a thickness of 30 nm to 100 nm and a thirdconductive oxide film (IrO_(z) film) 38 c having a thickness of 50 nm to150 nm. An oxygen composition y of the second conductive oxide film isset to a value near the stoichiometric composition (y=2), and an oxygencomposition z of the third conductive oxide film is set to a value lowerthan y, so that the advantages similar to the first embodiment can beobtained.

As shown in FIG. 3I, a first hard mask 45 of TiN and a second hard mask46 of SiO₂ are formed on the upper electrode 38. For example, the firsthard mask 45 is formed by sputtering and the second hard mask 46 isformed by CVD using O₂ and TEOS.

As shown in FIG. 3J, the second hard mask 46 is patterned to have a planshape of a ferroelectric capacitor to be formed. Next, by using thepatterned second hard mask 46 as an etching mask, the first hard mask 45is etched.

As shown in FIG. 3K, by using the second hard mask 46 and first hardmask 45 as an etching mask, the upper electrode 38, ferroelectric film37 and lower electrode 36 (and intermediate layer 34) are etched. Forexample, this etching is performed by plasma etching using mixture gasof HBr, O₂, Ar and C₄F₈. A composition of the etching gas can beselected depending upon an etching target. The patterned lower electrode36, ferroelectric film 37 and upper electrode 38 constitute aferroelectric capacitor 35, During this etching, the surface layer ofthe second hard mask 46 is also etched.

As shown in FIG. 3L, the second hard mask 46 is removed by dry etchingor wet etching. The first hard mask 45 is therefore exposed. Theswitching charge of the ferroelectric capacitor can be measured at thisstage because the first hard mask 45 of TiN and oxygen barrier film 33of TiAlN are conductive.

As shown in FIG. 3M, the oxygen barrier film 33, crystallinity improvedfilm 31 and underlying conductive film 30 are etched in the area wherethe ferroelectric capacitor 35 is not disposed. For example, as etchinggas, mixture gas of CF₄ gas of 5% at a flow rate ratio and O₂ gas of 95%is introduced into a down-flow type plasma etching chamber, and microwaves of 2.45 GHz are supplied to the upper electrode in the chamber ata high frequency power of 1400 W to perform dry etching. Alternatively,wet etching may be performed using mixture solution of H₂O₂, NH₂OH andpure water as etchant liquid. In this case, the first hard mask 45 lefton the upper electrode 38 is also removed and the upper electrode 38 isexposed.

As shown in FIG. 3N, a first protective film 50 of Al₂O₃ having athickness of 20 nm is formed on the exposed surface by sputtering.

As shown in FIG. 3O, recovery annealing is performed in an oxygenatmosphere in a temperature range of 550° C. to 700° C. Damages of theferroelectric film 37 can therefore be recovered. For example, if theferroelectric film 37 is made of PZT, it is preferable to performrecovery annealing for 60 minutes at a temperature of 650° C.

As shown in FIG. 3P, a second protective film 51 of Al₂O₃ having athickness of 20 nm is formed on the first protective film 50 by CVD,covering the ferroelectric capacitor.

As shown in FIG. 3Q, an interlevel insulating film 55 of SiO₂ having athickness of 1500 nm is formed on the second protective film 51 byplasma CVD using O₂, TEOS and He. Thereafter, the surface of theinterlevel insulating film 55 is planarized by CMP. The interlevelinsulating film 55 may be made of other inorganic insulating materialinstead of SiO₂.

After planarization, heat treatment is performed in a plasma atmosphereof N₂O gas or N₂ gas. This heat treatment removes moisture in theinterlevel insulating film and changes the quality of the interlevelinsulating film 55 so that moisture becomes hard to permeate into theinterlevel insulating film 55.

Thereafter, a barrier film 57 of AlO having a thickness of 20 nm to 100nm is formed on the interlevel insulating film 55 by sputtering or CVD.Since the underlying surface of the barrier film 57 is planarized,stable barrier performance can be retained more than the case whereinthe barrier film is formed on an irregular surface.

As shown in FIG. 3R, an interlevel insulating film 58 of SiO₂ having athickness of 800 nm to 1000 nm is formed on the barrier film 57 byplasma CVD using O₂, TEOS and He. The interlevel insulating film 58 maybe made of SiON or SiN instead of SiO₂. The surface of the interlevelinsulating film 58 is planarized by CMP.

As shown in FIG. 3S, via holes 80 are formed through five layers, fromthe interlevel insulating film 58 to the first protective film 50,reaching the upper electrodes 38 of the ferroelectric capacitors 35.

Heat treatment is performed at 550° C. in an oxygen atmosphere. Oxygenvacancies in the ferroelectric film 37 formed while the via hole 80 isformed can therefore be recovered.

A tight adhesion film of a Ti/TiN structure is formed on the innersurface of the via hole 80, and W or the like is buried in the inside ofthe via hole 80 to form a conductive plug 60. The tight adhesion filmmay be a two-layer structure of a Ti film formed by sputtering and a TiNfilm formed by MOCVD. After the TiN film is formed, a plasma process isexecuted using mixture gas of N₂ gas and H₂ gas to remove carbon in theTiN film. If the third upper electrode is made of Ir, it is possible toprevent the upper electrode 38 from being reduced, because the Ir filmprevents invasion of hydrogen. Since the oxygen composition of IrO_(y)of the second upper electrode 38 b is set to a value near thestoichiometric composition, a catalyzer function of the upper electrode38 relative to hydrogen is hard to present. The ferroelectric film 37 istherefore hard to be reduced by hydrogen radicals.

As shown in FIG. 3T, a via hole 85 is formed through seven layers, fromthe interlevel insulating film 58 to the oxidation preventive film 21,reaching the upper surface of the conductive plug 16. After a tightadhesion film of a Ti/TiN structure is formed covering the inner surfaceof the via hole 85, W or the like is buried in the inside of the viahole 85 to form a conductive plug 65.

Wirings 71 and 75 connected to the conductive plugs 60 and 65 are formedon the interlevel insulating film 58. First, sequentially formed are aTi film having a thickness of 60 nm, a TiN film having a thickness of 30nm, an AlCu alloy film having a thickness of 360 nm, a Ti film having athickness of 5 nm and a TiN film having a thickness of 70 nm. Thelamination structure constituted of these films is patterned to form thewirings 71 and 75. On this lamination structure, upper multilayerwirings of, e.g., second to fifth layers are formed.

FIGS. 4A and 4B are a SEM photograph showing a cross sectional structurein the state that the TiN hard mask layer and silicon oxide hard masklayer are formed on the ferroelectric capacitor structure, and aschematic cross sectional view showing the cross sectional structure. Avoids or defects structure having a large number of voids cannot beobserved in the upper electrode. A clearly crystallized flat upperelectrode can be obtained. Since the second and third upper electrodesare crystallized, it can be considered that the second and third upperelectrodes are not affected and re-crystallization will not occur evenif the hard mask layer is formed thereafter. Voids will not be formed inthe second upper electrode.

A switching charge of a ferroelectric capacitor of a fine cell array(0.7 μm×0.7 μm) manufactured by the embodiment method was 30.5 μC/cm²after ferroelectric capacitors were etched, and 30.1 μC/cm² after fivewiring layers were formed. Process deterioration did not occur.

With reference to FIG. 5, the third embodiment of the present inventionwill be described. Different points from the second embodiment will bedescribed mainly. By the processes shown in FIGS. 3A to 3C, W plugs 25are formed. With CMP, a recess 25 a is formed on the upper surface ofthe plug 25. An NH₃ plasma process is executed similar to the secondembodiment, and thereafter a Ti film is formed to a thickness of 100 nm.Heat treatment by RTA is performed in a N₂ atmosphere to nitridize theTi film. In this way, a TiN underlying conductive film 30 is formed. Theunderlying conductive film 30 may be made of TiAlN, tungsten, silicon orcopper not limiting only to TiN.

A depression is formed on the surface of the underlying conductive film30 by reflecting the recess. If the upper structure is formed in thisstate, there is a fear of crystallinity deterioration of a ferroelectricfilm. Therefore, the upper surface of the underlying conductive film 30is polished by CMP to planarize the surface and remove the depression,and the insulating film 22 is further polished. The upper surface of theinsulating film 22 is made flush with the upper surface of the plug. Onthis flat surface, a Ti conductive tight adhesion film is formed andnitridized to form a TiN film 31. Thereafter, an oxygen barrier film 34and a lower electrode 36 are formed. Further, RTA is performed at atemperature of 600° C. or higher in an inert gas. Processes similar tothe second embodiment continue thereafter.

With reference to FIG. 6, the fourth embodiment of the present inventionwill be described. Different points from the third embodiment will bedescribed mainly. Elements up to the conductive plugs 25 are formed. Inthis case, CMP is performed by using a low pressure polishing system. Norecesses are formed through polishing at a low pressure. Similar to thesecond embodiment, an NH₃ plasma process is executed to form a Ti filmhaving a thickness of 20 nm. Heat treatment by RTA is performed in an N₂atmosphere to nitridize the Ti film and form a TiN film 30. A TIAlNoxygen barrier film 33 and a lower electrode 36 are formed directly onthe TiN film. Processes similar to the second embodiment are executedthereafter.

In addition to sputtering and MOCVD, the ferroelectric film may beformed by a sol-gel method, metal organic decomposition (MOD), chemicalsolution deposition (CSD), CVD, or epitaxial growth. As theferroelectric film, a film may be used which changes a crystallinestructure to a Bi-containing layer structure or a perovskite structureby heat treatment. In addition to the PZT film, such a film includes afilm represented by a general formula of ABO₃ such as PZT, SBT, BLT, andBi-containing layer compounds finely doped with La, Ca, Sr, Si or thelike.

In forming the lowest layer of the upper electrode, reactive sputteringmay be performed using a target including one or more of, e.g.,platinum, iridium, ruthenium, rhodium, rhenium, osmium and palladium,under the condition that the metal of the platinum group is oxidized.

The present invention has been described in connection with thepreferred embodiments. The invention is not limited only to the aboveembodiments. It will be apparent to those skilled in the art that othervarious modifications, improvements, combinations, and the like can bemade.

1. A semiconductor device comprising: a semiconductor substrate; a MOStransistor comprising a gate electrode formed above said semiconductorsubstrate and current input/output regions formed in said semiconductorsubstrate on both sides of said gate electrode; a lower interlevelinsulating film formed on the semiconductor substrate, covering said MOStransistor; a ferroelectric capacitor formed above said lower interlevelinsulating film comprising: a capacitor lower electrode; an oxideferroelectric film formed on said capacitor lower electrode; a firstcapacitor upper electrode formed on said oxide ferroelectric film andmade of conductive oxide having a stoichiometric composition AO_(x1) andan actual composition AO_(x2); a second capacitor upper electrode formedon said first capacitor upper electrode and made of conductive oxidehaving a stoichiometric composition BO_(y1) and an actual compositionBO_(y2), where y2/y1>x2/x1; and a third capacitor upper electrode formedon said second capacitor upper electrode and having a compositioncontaining noble metal oxide of the platinum group; and a multilayerwiring structure formed above said lower interlevel insulating film,covering said ferroelectric capacitor and including interlevelinsulating films and wirings wherein said third capacitor upperelectrode is made of conductive oxide having a stoichiometriccomposition CO_(z1) and an actual composition CO_(z2), wherey2/y1>z2/z1.
 2. The semiconductor device according to claim 1, whereinA=B and x1=y1.
 3. The semiconductor device according to claim 1, whereinC=B and z1=y1.
 4. The semiconductor device according to claim 1, whereinsaid A, B, and C are ones selected from the group consisting fromplatinum, iridium, ruthenium, rhodium, rhenium, osmium and palladium. 5.The semiconductor device according to claim 4, further comprising: anupper interlevel insulating film formed on said lower interlevelinsulating film, covering said ferroelectric cpacitor; a first via holeformed through said upper and lower interlevel insulating films, andexposing one of said current input/output regions of said MOStransistor; a conductive plug filling said first via hole; second viaholes formed through said upper interlevel insulating film, and exposingsaid capacitor lower electrode and said third capacitor upper electrode;and local interconnects including one connecting said conductive plugand one of said capacitor lower electrode and said third capacitor upperelectrode.
 6. The semiconductor device according to claim 5, whereinsaid conductive plug is a W plug, and said local interconnect is made ofaluminum or aluminum alloy.
 7. The semiconductor device according toclaim 4, further comprising: a first via hole formed through said lowerinterlevel insulating film, and exposing one of said currentinput/output regions of said MOS transistor; a conductive plug fillingsaid first via hole; and an underlying conductive layer formed on saidlower interlevel insulating film and said conductive plug, and havingoxygen blocking function; wherein said capacitor lower electrode isformed above said underlying conductive layer.
 8. The semiconductordevice according to claim 7, further comprising an intermediate layerlocated between, and formed by reaction of said underlying conductivelayer and said capacitor lower electrode.